Method and apparatus for the broadbanding of power type velocity modulation electron discharge devices by interaction gap spacing



C. E. BLINN ETAL FOR THE BROADBANDING OF POW ULA'IION ELECTRON DISCHARGE DEVICES BY INTERACTION GAP SPACING Filed May 25, 1960 METHOD AND APPARATUS TYPE VELOCITY MOD Oct. 5, 1965 W 5. r Wm m w \K/ N: m .WNMN MN a f A J Q11 22 Q NEE 53% UN 1 m IQ rm 5 m l N og M Wm WM M 0 .M NU UI N MN 0% m m 0 Q M N N E N WWW N LM\ I: INN? EL 0% United States Patent 3,210,593 METHOD AND APPARATUS FOR THE BROAD- BANDING OF POWER TYPE VELUCHY MUDU- LATION ELECTRUN DESCHARGE DEVICES BY INTERACTION GAP SPACING Charles E. Blinn, Redwood City, and George Caryotakis, Los Altos, Calif., assignors to Varian Associates, Paio Alto, Calif., a corporation of California Filed May 25, 1960, Ser. No. 31,755 9 Claims. (Cl. SIS-5.43)

This invention relates in general to novel electron tube apparatus of the type employing cavity resonators such as high power klystron tubes useful, for example, as power amplifiers in transmitter systems and as power sources for linear accelerators.

More particularly, the invention relates to high frequency tubes of the above type which may be operated over a wide band of frequencies without requiring mechanical tuning adjustments as, for example, where input signals of varying frequency are applied to the tube in rapid succession. Heretofore, electron tubes in high power applications of high frequency broad band systems capable of handling an instantaneou power as large as 4 megawatts and an average power as large as several kilowatts consisted of a number of resonant cavities which were stagger-tuned in a manner similar to that used in low frequency cascade amplifiers. The amplification mechanism of this invention depends on velocity modulation introduced on the electron beam by RF. voltage across the interaction gap of a cavity. In the drift space down stream of the gap, the electrons bunch and produce density modulation which will excite a subsequent cavity, introducing an additional, and larger, velocity modulation on the beam. The original modulation, however, persists and this distinguishes velocity modulation tube theory from simple cascade amplifier theory. At the output, where the modulations are finally summed, the beam contains modulation components due to all preceding cavities. The output circuit converts the modulated beam current into RF. energy.

In order to'take advantage of considerable bandwidth capabilities in the driver section, it has been found that the bandwidth can be increased on the order of two times that previously possible, with a single interacting cavity, by coupling a second synchronously tuned cavity to the interacting cavity. A double tuned output of this type is disclosed in the co-pending US. Patent application of Robert L. Jepson and Richard L. Walter, Serial No. 784,494, filed January 2, 1959, entitled, High Frequency Tube Apparatus and Coupled Cavity Output Circuit Therefor and now issued as US. Patent 3,028,519 on April 3, 1962, to which reference is made for a more detailed description. Thus, a great need has now arisen for high power klystron tubes in which the driver section would be able to produce a wider bandwidth while maintaining optimum gain.

It is accordingly the object of this invention to provide a high frequency tube apparatus of the electron beam velocity modulation type employing standard tuned reentrant type resonant cavities, including klystrons having a double tuned output circuit, reliable over a wide band of frequencies at both high peak power and high average power output levels.

One feature of the present invention is the provision of increasing the gap length between the re-entrant drift tubes of the cavities thereby lowering the beam-loading Q of each cavity.

Still another feature of the present invention is a wide band fixed-tuned pulse amplifier klystron to provide wide bandwidth with high gain and high efiiciency and requiring no tuning or RF. adjustment.

These and other features and advantages of the present invention will become apparent upon perusal of the following specification taken in connection with the accompanying drawing wherein,

FIG. 1 is a side view showing the general construction of a novel high power klystron tube in accordance with the present invention with the center of the tube cut away and the protective cover removed,

FIG. 2 is a graph showing power output in megawatts versus frequency in mcs., and

FIG. 3 is an enlarged cross-sectional View of the portion of the structure of FIG. 1 taken along line 3-3 in the direction of the arrows.

Referring now to FIGURES l, 2 and 3 of the drawings, a segmented tubular cathode assembly 11 provides a beam of electrons which is projected longitudinally within the tube apparatus. The cathode assembly 11 includes all the well known components necessary to derive a high negative beam pulsating voltage and apply it between the electron gun housing and a grounded anode for accelerating electrons of the beam in a known manner and therefore is not shown or described in detail.

Spaced along the electron beam path are a plurality of driver cavity resonators 13, 14, 15, and others (not shown) which are positioned between cavity resonators 14 and 15. Each of the cavity resonators is centrally apertured to receive the beam passable therethrough to form an interaction gap within the respective cavities between the opposed ends of adjacent drift tube segments 18 and 19.

Electromagnetic energy which is to be amplified is fed into the input cavity 13 by means of a vacuum-sealed coaxial connector 21 and coupling loop 22. The connector 21, in turn, is adapted for external connection through a coaxial line 23 and an externally mounted panel jack 24 to a driver tube, not shown.

The electron beam passes through output cavity 25 and terminates in a water cooled electron collecting structure 28, the details of which are well known and hence not described in detail in the present application.

The output cavity circuit in the present invention consists of a rectangular-shaped cavity resonator 25. The output end of cavity 25 is formed by iris plate 27. The cavity 25 is coupled through iris 27 to auxiliary cavity resonator 26, which is placed between the coupling iris plate 27 and an external iris plate 30 which couples to the output waveguide structure 29.

A hollow tuning stub 26 is inserted into cavity resonator 26 through an aperture in one side thereof. A metal plug 20 is brazed into the hollow portion of tuning stub 26' to seal off the tube.

The output energy is propagated from auxiliary output cavity 26 through iris 30 along a waveguide 29 which is provided with an impedance matching transformer 31 and a plurality of stainless steel stiffening members 32 which guard against waveguid buckling due to pressure difl'erentials experienced upon exhaustion of the tube. The output energy passes from waveguide section 29 to a higher impedance section of circular waveguide stnicture 33 having midway therein a gas tight wave permeable window 35 as of, for example, aluminum oxide which is sealed in a vacuum tight manner to a thin metallic ring 34 as of, for example, copper, thus allowing for the thermal expansion and the contraction of window 35 in the ring 34 without breaking the seal between these members. In order to provide transmission over a wide band of frequencies it is found preferable to make the circular waveguide section including window structure electrically in the order of /2 wavelength long at the center frequency of the passband. The circular waveguide 33 communicates with output flange 38 through waveguide 37. The flange 38 mates with a standard waveguide.

Proper alignment and rigidity of the apparatus under adverse shock, vibration and temperature conditions are maintained by stiffener rods 40 and stiffener plates 43. Stiffener rods 40 and stiffener plates &3 reduce any microphonic tendencies and improve the electrical stability of the tube at high ambient temperatures.

The stiffener rods 40 terminate in magnetic pole pieces 44 and 45 which are adapted to accommodate an external magnet structure (not shown) in which the tube is mount- 'ed. The external magnet provides a flux path bounded by pole pieces 44 and 45 giving rise to a magnetic field for focusing the electron beam as its passes from the cathode through the various cavities and drift tubes to the collector 28. This section of the tube is preferably enclosed by a protective cover 46 as of, for example, aluminum. Pole pieces 44 and 45 are constructed of a material of a high magnetic permeability such as, for example, iron. On the other hand in the area between the pole pieces the magnetic field should not be perturbed and thus, Where possible, parts should be made of materials possessing no magnetism. For example, stiffener rods 40 and stiffener plates 4-3 could be constructed of a non-magnetic variety of stainless steel.

A plurality of tuner shafts 47 are provided to permit tuning of the cavity resonators from a remote station safe from the harzards of X-radiation. Operation of the tuner shafts and the cavity resonators is well known in the art and therefore not described in the present disclosure.

Fluid cooling of the tube is provided through fluid fittings 51 and 52 which lead respectively to inlet manifolds 53 and 54 mounted on opposite sides of electron collector 28. The fluid passes from inlet manifolds 53 and 54 and from there flows throughout the entire tube system through a series of cooling channels and cooling conduits.

Broad band multi-cavity klystrons usually require low and sometimes also unequal Qs in the cavities. This has previously been done, for example, either by including a lossy material inside the cavity, or by coupling power out of the cavity. A new approach to this problem, however, is being used in the present invention. For the first time this tube utilizes beam loading to obtain the desired Qs.

The shunt resistance to Q ratio of the cavity and the beam loaded admittance of the cavity both increase approximately linearly with gap spacing. Therefore, for high power klystrons where the loaded Q of the cavity heavily depends upon beam loading only, i.e., the beam loading is about equal to cavity losses plus external loading, the Q of the cavity varies roughly as the inverse square of the gap spacing. Thus, the desired Q may be obtained simply by properly dimensioning the gap spacing in each cavity during manufacture.

In order to obtain the broadband performance required of the present tube, it was seen that extremely low beam loaded Qs were needed. These low Qs in the present invention, are realized entirely by the use of beam loading. It has been found that it is possible to vary the beam loading Q over a range of more than 2-1 by varying the gap length between the ends of the mutually opposed drift tubes. It has been found, for example, that a gap length of slightly less than 1 radian of electronic drift angle at the operating frequency of the tube has a beam loaded cavity Q of 150. Electrical drift angle as used above is the same thing as electron transit time or angle. See Spangenberg, Vacuum Tubes, 1948, page 551, which provides a nomograph for deter mining the drift space or interaction gap length at the frequency of interest in frictions of a wavelength given the transit angle in radians and the beam voltage in volts. In a cavity of the same type a gap length of 1 /2 radians gave a beam loaded Q of 70. If desired the cavity Qs could be reduced still further by increasing the gap lengths. I

It has been found that in a six-driver cavity, sevengap, stagger-tuned, double-tuned output klystrons, with all gap spacings set for 1.5 radians a bandwidth of 6% was produced, at the half-power point with a peak efficiency of 47% and a gain of 45 db. FIG. 2 shows the power output in megawatts versus frequency in megacycles of a tube incorporating the increased gap spacing of the present invention.

The main advantages of the beam loaded cavity are that no output windows for external loading are required and that no provision for R.F. power dissipation inside the vacuum envelope must be provided and the inherent simplicity of the construction of this device is of particular value in power tubes of the present type where artificial loading of cavities preceding the output cavity is difiicult because of the large R.F. power present.

In the present tube, the input cavity is chosen as the lowest frequency cavity. This is done to improve the bandwidth of the response. In tuning the cavities in a power klystron tube of the type of the present invention the optimum choice of cavity tunings may be generalized as follows: (1) the input cavity should be tuned to the low end of the band to increase the bandwidth, (2) the last one or two cavities should be definitely tuned to the high frequency end to enhance efficiency, and (3) the cavities with the lowest Qs should be nearer to the input cavity.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The method of obtaining a broadband response in an electron discharge amplifier of the high power type having a plurality of re-entrant type driver cavity resonators for velocity modulation of a beam passing therethrough wherein the loaded cavity Qs are heavily dependent upon beam loading comprising the stops of: distributing the resonant frequencies of the individual driver cavities over the desired operating band of frequencies, beam loading the individual driver cavities by dimensioning the gap spacing between the successive drift tubes to be more than 1 radian in length, tuning the input cavity to the low end of the frequency band, and tuning the last driver cavities to the high frequency end of the frequency band.

2. In a broadband klystron amplifier tube, the combination comprising a driver section for producing a current modulation on an electron beam passable therethrough, said driver section including a plurality of cavity resonators which are tuned to a set of frequencies distributed over a band of frequencies which at least partially includes the desired passband of the tube, a plurality of axially aligned drift tube segments interconnecting said cavity resonators, the axial spacing between said drift tube segments and within said cavity resonators making up the gap space in which the electron beam passable through said drift tube segments interacts with the fields of said cavity resonators, said gap spacing being dimensioned more than 1 radian in length in order to lower the Q of each resonant cavity thereby increasing the bandwidth of said klystron amplifier tube, an output circuit for extracting energy from said current modulated beam said output circuit comprising a first cavity resonator tuned near the center of the bandpass and containing an electron beam interaction region therein and a second cavity resonator tuned near the center of the bandpass and coupled to said first cavity resonator and means for coupling said second resonator to an external power utilization load.

3. The method of obtaining a broadband response in an electron discharge amplifier of the high power type having a plurality of re-entrant type driver cavity resonators for velocity modulation of a beam passing there through wherein the loaded cavity Qs are heavily dependent upon beam loading comprising the steps of beam loading the individual driver cavities by dimensioning the gap length between the successive drift tubes to more than 1 radian in length thereby effecting greater interaction between the electron beam passing through the drift tubes and the cavity resonators to lower the Q of the driver cavities thereby increasing the bandwidth of the tube.

4. A broadband electron discharge amplifier of the high power type having a plurality of re-entrant type driver cavity resonators for velocity medulation of the beam passing therethrough wherein the loaded cavity Qs are heavily dependent upon beam loading comprising, means producing :a current modulation on an electron beam including a plurality of cavity resonators, a plurality of axially aligned drift tube segments interconnecting said cavity resonators, fixing the space between said plurality of axially aligned drift tubes defining a gap space, said gap space being dimensioned in length to more than one radian thereby lowing the Q of each resonant cavity to increase the bandwidth of said amplifier tube.

5. A method of obtaining a broadband response in a multicavity high power klystron amplifier tube wherein the loaded cavity Qs are heavily dependent upon beam loading comprising the steps of beam loading the individual driver cavities by dimensioning the gap length between the drift tubes to greater than one radian thereby increasing the bandwidth of the klystron tube.

6. A broadband electron discharge amplifier of the high power type having a plurality of re-entrant type driver cavity resonators for velocity modulation of the beam passing therethrough wherein the loaded cavity Qs are heavily dependent upon beam loading including, cathode means for forming and projecting a high power beam of electrons over a predetermined beam path, collector means disposed at the terminal end of said beam path for collecting the electron beam and for dissipating the energy thereof, a plurality of re-entrant driver cavities apertured for the passage of the electron beam therethrough and disposed along the beam path in electromagnetic energy exchanging relationship therewith, said re-entrant portions of said driver cavities defining the energy exchanging gaps within the cavity resonators, means forming an output structure disposed along the beam path down stream from said driver cavities in energy exchanging relationship with said beam for extraction of wave energy from said beam, means forming an input for applying signal energy to said beam up stream of said output means, said plurality of driver cavities being tuned to different frequencies within the band of frequency of the broadband tube, and at least one of said re-entrant driver cavities having an interaction gap length in excess of 1 radian for beam loading of said driver cavity to obtain broadband amplification.

7. The apparatus according to claim 6 wherein said plurality of driver cavities include at least one cavity tuned to the high frequency end of the frequency band of the tube, and a second driver cavity disposed up stream of said high frequency driver cavity tuned to the low end of the frequency band, and said second low frequency driver cavity having an interaction gap length in excess of 1 radian.

8. The apparatus according to claim 7 wherein the gap length of said second low frequency driver cavity is at least 1.5 radians in length.

9. The apparatus according to claim 8 wherein all of said driver cavities include interaction gaps of a length at least 1.5 radians in length and said second low frequency driver cavity having a loaded Q less than 100.

References Cited by the Examiner UNITED STATES PATENTS 2,439,831 4/48 Varian et al. 3155.49 2,515,280 7/50 Varian 315-548 2,529,668 11/50 Wang 3155.48 2,934,672 4/60 Pollack et al. 3155.46

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS, Examiners. 

4. A BROADBAND ELECTRON DISCHARGE AMPLIFIER OF THE HIGH POWER TYPE HAVING A PLURALITY OF RE-ENTRANT TYPE DRIVER CAVITY RESONATORS FOR VELOCITY MEDULATION OF THE BEAM PASSING THERETHROUGH WHEREIN THE LOADED CAVITY Q''S ARE HEAVILY DEPENDENT UPON BEAM LOADING COMPRISING, MEANS PRODUCING A CURRENT MODULATION ON AN ELECTRON BEAM INCLUDING A PLURALITY OF CAVITY RESONATORS, A PLURALITY OF AXIALLY ALIGNED DRIFT TUBE SEGMENTS INTERCONNECTING SAID CAVITY RESONATORS, FIXING THE SPACE BETWEEN SAID PLURALITY OF AXIALLY ALIGNED DRIFT TUBES DEFINIGN A GAP SPACE, SAID GAP SPACE BEING DIMENSIONED IN LENGTH TO MORE THAN ONE RADIAN THEREBY LOWING THE Q OF EACH RESONANT CAVITY TO INCREASE THE BANDWIDTH OF SAID AMPLIFIER TUBE. 